The subject invention is broadly directed to indicators for electric storage batteries which provide a visual indication of the life and charge of the battery, and more particularly the invention is directed to a battery condition indicator that is both inexpensive and capable of surviving very adverse conditions during the expected life of the battery.
The invention has its primary application in combination with an electric storage battery used to power railroad signalling equipment. Such equipment is subjected to severe mechanical and thermal shocks but must operate with high reliability. The battery in this particular application is detachably connected to a telemetering transmitter which is located at the rear of a train. Since the data transmitted is critical to the safe operation of the train, it is important that the battery be fresh and properly charged. It is of course well known that as batteries age, they become less reliable as a source of electrical power. This is usually manifested by a failure of the battery, and the condition is remedied by replacement of the battery. However, in critical applications such as railroad signalling, it is necessary to anticipate battery failure and replace the battery before the failure occurs. More commonly, it is known to provide some indication of the current state of the charge of the battery, and this is usually done by means of any number of well known instruments including voltmeters, ammeters and hygrometers.
One example of battery condition indicators known in the prior art is disclosed in U.S. Pat. No. 4,413,221 to Benjamin et al. This patent discloses a circuit for both determining the capacity of a battery while charging the battery and the state-of-charge of the battery by measuring the voltage level during a discharge interval. In this way, both short term and long term indications of the battery are provided. However, the Benjimin et al system is quite complex and not suitable to applications requiring rugged and inexpensive indicators.
U.S. Pat. Nos. 3,118,137 to Vincent, 4,027,231 to Lohrmann, and 3,778,702 to Finger each show different examples of battery charge indicators which provide an indication of the short term condition of the battery. U.S. Pat. Nos. 4,151,454 to Iida, 4,163,186 to Haley and 4,380,726 to Sado et al are representative of remaining service life indicators for electric storage batteries. Thus, although it is well known in the prior art to provide battery condition indicators which show the short term condition, i.e. charge, of the battery and the long term condition, i.e. life remaining or age, of the battery, what is needed is a simple and inexpensive way to provide indications of battery conditions which will survive in a hostile environment such as experienced in railroads.